Aqueous reduction of cobalto-cobaltic oxides in ammoniacal medium using ammonium sulphite as the reductant

Studies have been carried out on aqueous reduction of cobalto-cobaltic oxide in ammoniacal medium using ammonium sulphite as the reductant. The various parameters studied include agitation speed, leaching time, ammonia and ammonium sulphite concentrations. The kinetics of dissolution follow a surface-controlled model. The order of reactions with respect to both ammonia and ammonium sulphite were found to be first order. The XRD patterns and TG-DTA traces of the original cobaltocobaltic oxide and a typical leach residue were identical, showing that the reaction is surface-controlled and no preferential dissolution of Co(II) or Co(III) takes place. It was observed that on cooling and storing leach solutions obtained at 391 K using ammonia more concentrated than 4.5 and 0.23 M ammonium sulphite resulted in precipitation of a crystalline compound which showed distinct lines in the XRD pattern but did not correspond to any known compound of Co. The chemical analysis, TGDTA and FTIR analysis suggest the compound to be Co(NH3)4S2O6·2H2O.